Low bit rate voice coding has been performed through use of signal bandwidth limitation, whereby the original voice signal is first filtered to derive therefrom a base-band signal which, according to Nyquist theory could be sampled efficiently at a rate lower than the rate used for the original full-band signal. Said limited bandwidth may therefore be coded at low bit rate.
Subsequent decoding and conversion back to the original signal is achieved by spreading the base-band over a broader bandwidth and up-rating the sampling rate.
Traditionally, the above mentioned filtering is achieved with a low pass filter with a cut-off frequency at about 1300 Hertz, i.e. large enough to include any speaker's pitch frequency. Said low pass filtering is either operated directly over the signal provided by the voice terminal, or operated over a decorrelated residual derived signal from said voice terminal signal. Both cases may be defined as dealing with voice terminal derived signals.
In some applications, e.g. related to telephony, the network over which the coded voice signal is to be transmitted, is also used to carry non voice originated signals, like for instance busy tones or other service tones. Said tones are made of a pure sinewave which might be at a frequency higher than the low-pass filter cut-off frequency.
The conventional base-band coding operations would then lead to loss of tones, or even worse, to dramatic tone distortions which could affect the whole network operation.